Volumetric pump with pump plunger support and method

ABSTRACT

A volumetric pump and method for displacing a predetermined quantity of fluid at a predefined cracking pressure, independent of supply and output pressures. The volumetric pump (30) includes an inlet cracking valve (46), an outlet cracking valve (52), and a plunger (48) for displacing fluid a pumping from a pumping portion (34b) of flexible tubing (34) that extends through the volumetric pump. The pumping portion of the flexible tube fills with liquid when the inlet cracking valve is fully opened and is urged to expand by jaws (236) on pivotally-mounted arms (234). The arms are forced to pivot, as tubing reshaper rollers (160), disposed on the plunger, roll along the inner surface (232) of each arm. During a pumpback-pressurization segment of the pumping cycle, the inlet cracking valve applies a cracking force to the flexible tubing, while the plunger compresses the pumping portion of the flexible tubing sufficiently to develop a cracking pressure that displaces excess fluid back through the inlet cracking valve toward a container (32). After the excess fluid has been forced from the pumping portion of the flexible tubing, a pumping segment of the cycle begins, wherein the inlet cracking valve closes fully and the outlet cracking valve applies a cracking force to compress the flexible tubing. Fluid is then forced by the plunger from the pumping portion of the flexible tubing into a distal portion (34c). The volumetric pump compensates for variations in elasticity of the flexible tubing that would otherwise cause variations in the cracking pressure, using balance blocks (42 and 58). A volumetric pump (400) for use with a cassette (300) comprises a second embodiment of the volumetric pump, in which inlet and outlet cracking valves are defined in respect to the forces applied by actuators against a flexible membrane (340) in the cassette, while a plunger (326) displaces fluid from a pumping chamber (360) in the cassette. Volumetric pump (400) applies appropriate forces to an inlet valve actuator (318) and an outlet valve actuator (332) to achieve cracking and closure forces, just as in the first embodiment.

This is a divisional of now U.S. Pat. No. 5,158,437, the priorapplication Ser. No. 742,623, filed Aug. 8, 1991, which in turn is adivisional of U.S. Pat. No. 5,055,001, issued Oct. 8, 1992, the benefitof the filing dates of which are hereby claimed under 35 U.S.C. § 120.

TECHNICAL FIELD

This invention generally pertains to a positive displacement volumetricpump, and specifically, to a volumetric pump in which a plunger acts ona flexible member to displace fluid from a chamber.

BACKGROUND OF THE INVENTION

Intravenous infusion of medicinal liquids has traditionally beenaccomplished using drip regulated, gravity flow systems. However, it isgenerally recognized that a more precisely regulated flow of drug to thepatient can be administered with a pump. Because of their simplicity andease of use, peristaltic pumps are often used for this purpose. Agravity flow system is readily converted to a peristaltic pump infusionsystem by threading the drug delivery or intravenous (IV) line, which isattached to the drug container, through the peristaltic pump. The pumpthen controls the rate at which the drug is delivered to the patient.

A peristaltic pump displaces liquid by repetitively compressing asection of the flexible tube comprising the IV line. This line isthreaded through a channel formed in the pump and extends unbroken, fromthe drug container to the patient. In one type of peristaltic pump, thechannel is curved around a central axis. A rotating arm with rollersfixed on each end compresses the section of tubing disposed within thechannel, advancing the rollers along the longitudinal axis of the tubingas the arm rotates about the central axis. Liquid within the tubing isthus forced ahead of the advancing roller along the internal passage inthe line.

Another type of peristaltic pump has a linear channel in which the IVline is threaded and is thus referred to as a linear (or traveling wave)peristaltic pump. The linear peristaltic pump includes a plurality offinger-like plungers that are sequentially actuated by cams mountedalong a motor driven shaft. Liquid within the section of tubing thatextends along the linear channel is advanced along the tubing'slongitudinal axis by the advancing wave-like compression of the fingers.An example of such a pump is disclosed in U.S. Pat. No. 4,479,797.

Inlet and outlet valves and a single liquid displacement plunger areused in another type of peristaltic pump. Each pumping cycle in thistype of pump begins with the outlet valve closed and the inlet valveopen. Fluid flows from the source container into a short section oftubing that is disposed between the inlet and outlet valve. After thissection of tubing has filled with liquid, the inlet valve closes and theoutlet valve opens. The plunger then compresses the short section oftubing between the valves, displacing the liquid contained therein, andforcing it from the pump. U.S. Pat. No. 4,559,038 discloses aperistaltic pump of this type.

Cassette pumps are also frequently used in administering medicinalfluids to a patient and normally provide a more accurate rate of fluidflow than a peristaltic pump. In a cassette pump, a cassette comprisinga plastic housing that includes a pumping chamber and inlet and outletvalves, is connected via a disposable tube set to a drug container. Thecassette is inserted into an appropriate device designed to drive thecassette and administer fluid at a controlled rate. The pumping deviceincludes an inlet valve actuator, an outlet valve actuator, and apumping plunger. Inside the cassette, passages connect the inlet valveand the outlet valve to the pumping chamber; a flexible membrane, whichis sealed between two halves of the plastic housing, interrupts fluidflow through inlet and outlet valve openings formed in the housing whenthe membrane is deformed by the inlet and outlet valve actuators. Theplunger acts on the flexible membrane covering the pumping chamber inthe cassette to force liquid past the open outlet valve and through anoutlet port of the cassette. An example of a cassette pump is disclosedin commonly assigned U.S. Pat. No. 4,818,186.

The rate at which fluid is delivered by each type of positivedisplacement pump discussed above is normally controlled by the rate atwhich the pump operates, e.g., the rotational rate of the rotating armin that type of peristaltic pump. Furthermore, the accuracy with which agiven rate or volume of fluid flow can be achieved by these pumps isdependent upon the pressure of the fluid at the input of the pump andthe back pressure at its output. Since both the flexible tubing (in theperistaltic pumps) and the flexible membrane (in the cassette) define acompliant pumping chamber, the volume of fluid that fills the pumpingchamber is affected by the head pressure of the fluid from the drugcontainer. Similarly, the volume of fluid delivered at the output of thepump depends on the back pressure of the fluid downstream of the outlet.The cassette pump and the PG,5 single plunger type of peristaltic pump,both of which have positive closure inlet and outlet valves, areparticularly sensitive to head and back pressure because the volume ofthe pumping chamber disposed between the valves and the amount of fluidthat fills the chamber generally must be constant to provide an accurateand consistent rate of flow from the pump.

Several other parameters can affect the accuracy of fluid flow deliveredby specific types of positive displacement pumps. For example, when thecompression force is removed from the tubing in a peristaltic pump, thetubing must recover to a defined and consistent internal diameter toinsure that the same volume of fluid is delivered in each pump cycle. Ifthe volume of the passage defining the pumping chamber changes overtime, for example, due to changes in the tubing elasticity, the pump'sflow rate will also change. Inexpensive polyvinyl chloride (PVC) tubing,commonly used for disposable tube sets in medical IV applications, isknown to experience changes in elasticity over time and with repetitivecompression of the tubing, thereby affecting the extent to which thetubing recovers when a compression force is removed.

In the single plunger type of peristaltic pump, the plunger shouldcompress the tubing uniformly and consistently with each pumping stroketo provide an accurate and consistent rate of fluid flow from the pump.The plunger mounting assembly must permit the plunger to move freelyback and forth along a reciprocation axis, yet should prevent it fromtwisting or moving laterally away from this axis, because such movementcan change the compression stroke volumetric displacement. Since theplunger is typically driven by a rotating cam, the mounting assemblyshould also provide a biasing force to maintain the plunger in contactwith the cam surface, preferably without introducing sliding friction orusing helical springs. Most prior art plunger mounting assemblies do notaddress all of these concerns.

Due to the potential safety concerns involved in administering medicinalfluids intravenously to a patient, an infusion pump should include anair-in-line sensor to detect large air bubbles within the pump and stopthe pump before such bubbles are infused into the patient's circulatorysystem. Provision should also be made to detect when a drug containerbecomes empty or a supply line connected to the container occluded. Ifthe flow of fluid from the pump is interrupted for any reason, the pumpshould be shut off and an alarm sounded to alert medical personnel.Ideally, these functions should be integrated into the pump, and are insome prior art pumps. However, virtually none of the availableperistaltic pumps currently provide both of these safety-relatedfeatures.

In consideration of these problems that exist with the prior art pumps,it is an object of the present invention to provide a positivedisplacement pump in which the volume and rate of delivery of fluid fromthe pump is substantially unaffected by variations in the pressure offluid supplied to the pump. Another object of this invention is toprovide a pump in which the volume and the rate fluid is delivered fromthe pump is substantially unaffected by variations in fluid pressuredownstream of the pump. Yet a further object is to provide a positivedisplacement fluid pump that delivers fluid to an output port of thepump at a predefined pressure. Still a further object of the inventionis to provide a spring-biased support for a plunger enabling it toreciprocate freely along a reciprocation axis without sliding friction,while preventing it from twisting or moving laterally away from thereciprocation axis. These and other objects and advantages of thepresent invention will be apparent from the attached drawings and theDescription of the Preferred Embodiments that follows.

SUMMARY OF THE INVENTION

In accordance with the claimed invention, a volumetric pump is adaptedto pump a fluid through a set that includes a flexible member, bydeforming different portions of the flexible member in a predefinedpumping cycle. The portions of the flexible member respectively define achamber, an inlet passage, and an outlet passage. Deformation of theportion of the flexible member defining the chamber reduces the volumeof the chamber and displaces fluid from it. The volumetric pump includesa chassis on which an inlet valve is disposed adjacent to the inletpassage. The inlet valve is positioned to act on an inlet portion of theflexible member to control fluid flow through the inlet passage andexerts a first and a second sealing force on the inlet portion of theflexible member during the pumping cycle. The first sealing force issubstantially less than the second sealing force and allows fluid toflow through the inlet passage from the chamber as fluid in the chamberis initially pressurized beyond a predefined level. The second sealingforce subsequently interrupts fluid flow through the inlet passage fromthe chamber, enabling fluid pressurized in the chamber to be forcedthrough the outlet passage.

A plunger disposed on the chassis adjacent the chamber is operative todeform a pumping portion of the flexible member to displace fluid fromwithin the chamber. An outlet valve is disposed on the chassis adjacentthe outlet passage and is thus positioned to act on an outlet portion ofthe flexible member to control fluid flow through the outlet passage.The outlet valve exerts a forward-flow sealing force and a back-flowsealing force on the outlet portion of the flexible member during thepumping cycle. The forward-flow sealing force is substantially less thanthe back-flow sealing force and allows fluid to flow through the outletpassage from the chamber; however, the back-flow sealing forcecompletely interrupts fluid flow past the outlet valve. Means are alsoprovided for actuating the inlet valve, the outlet valve, and theplunger to deform the portions of the flexible member during the pumpingcycle, thereby forcing fluid through the outlet passage at apredetermined rate of fluid delivery.

The means for actuating the valves and plunger comprise a motor, aninlet valve cam, an outlet valve cam, and a pumping cam. A profile onthe pumping cam sequentially defines a filling segment, apumpback-pressurization segment, and a pumping segment. After thechamber is filled with fluid, the inlet valve exerts the first sealingforce and the outlet valve blocks fluid flow from the chamber into theoutlet passage, enabling fluid pressurized in the chamber to be forcedthrough the inlet passage. The motor rotates the pumping cam from astart position and the pumping cam actuates the plunger to pressurizefluid in the chamber to the predefined level, thus forcing excess fluidin the chamber to backflow through the inlet passage until the pumpingcam reaches a rotational position corresponding to the start of apumping stroke. During the pumping stroke, fluid is again displaced fromthe chamber by the plunger and forced to flow through the outlet passageas the inlet valve cam causes the inlet valve to close with the secondsealing force, thereby preventing fluid flow through the inlet passage.After the pumping stroke is completed, the inlet valve cam causes theinlet valve to open fully, enabling fluid to flow from a source throughthe inlet passage, again filling the chamber.

At all times, even when the volumetric pump is not pumping fluid, atleast one of the inlet and outlet valves blocks fluid flow through theinlet and outlet passages, respectively. The rate at which the motorrotates the inlet, the outlet, and the pumping cams, at least in part,controls the rate at which the fluid is delivered through the outletpassage. To further reduce the rate of fluid delivery, the motor stopsrotating the pumping cam for an interval of time at least once duringthe pumping stroke.

In one embodiment of the volumetric pump, the flexible member comprisesa tube and the pumping chamber comprises a portion of the tube disposedbetween the inlet and the outlet valves. In this embodiment, the plungerpartially compresses the tube when the pumping chamber is filled withfluid at the start position of the pumping cam. The volumetric pumpfurther comprises tube-shaping means that are disposed where the plungercompresses the tube and are operative to bias the tube to morecompletely fill with fluid.

In another embodiment of the volumetric pump, the flexible membercomprises a generally planar, elastomeric membrane disposed in ahousing. The inlet valve and the outlet valve comprise spring-biasedmembers that act on the membrane to control fluid flow through passagesin the housing.

The volumetric pump may include means for determining if fluid isflowing through the outlet valve as the plunger compresses the flexiblemember. It may also include means for sensing a fluid pressure upstreamof the inlet valve and/or downstream of the outlet valve. Due to thecompressibility of a gaseous fluid, if the chamber is substantiallyfilled with such a fluid, the pressure developed in the chamber duringthe pumping cycle is insufficient to force the gaseous fluid past theoutlet valve.

A method for preventing variations in a supply pressure from affecting afluid delivery rate from a pump that includes an inlet valve and anoutlet valve, and which displaces fluid from a pumping chamber definedby a flexible member is another aspect of the present invention. Themethod comprises steps generally consistent with the functionsimplemented by the elements of the volumetric pump, as described above.

Yet a further aspect of this invention is an apparatus for supporting areciprocating plunger in a positive displacement pump. The apparatusincludes a frame having two spaced-apart members disposed on oppositesides of the plunger. A plurality of pairs of flexures extend betweenthe two spaced-apart members and the plunger, each flexure having alongitudinal axis. The flexures comprising each pair of flexures arealigned substantially parallel to each other. Due to the disposition andspacing of the flexures, the plunger is supported so that it canreciprocate along a reciprocation axis that is generally normal to thelongitudinal axes of each pair of flexures, but is constrained by thepairs of flexures so that it does not move transversely in respect tothe reciprocation axis.

The flexures are each attached to the spaced-apart members and to theplunger and apply a biasing force to the plunger that is directed alongthe reciprocation axis. A first pair of the flexures are attached to theplunger at a first level along the reciprocation axis, and a second pairare attached to the plunger at a second level. The first pair of theflexures are spaced apart a first distance, while the second pair arespaced apart a second distance that is substantially different than thefirst. The flexures bias the plunger along the reciprocation axisagainst a drive force that displaces the plunger along the reciprocationaxis. One pair of the flexures is preferably spaced apart from the otherpair so as to prevent rotation of the plunger about a rotation axis thatis normal to the reciprocation axis. Each flexure preferably comprisesan elongate flat metal spring.

A method of constraining a plunger to move only along a reciprocationaxis, preventing it from twisting or moving laterally away from thereciprocation axis, is yet another aspect of the present invention. Themethod includes steps generally consistent with the functions of theelements of the apparatus for supporting a plunger, as described above.

Still another aspect of the present invention is apparatus forcompressing and shaping a flexible tube. This apparatus includes plungermeans, mounted to move bidirectionally along a reciprocation axis. Theplunger means periodically compress the flexible tube to displace afluid from an interior passage disposed within the flexible tube andthen allow the flexible tube to expand to at least a partiallyuncompressed condition. Drive means are included for periodicallydriving the plunger means to move along the reciprocation axis. Tubeshaper means are actuated by the motion of the plunger means and areoperative to apply a reshaping force against the flexible tube thattends to expand the interior passage to a maximum desired volume.

The tube shaper means comprise a pair of arms that are pivotally mountedon opposite sides of the plunger means. Jaws are disposed on these arms,at each side of a segment of the flexible tube compressed by the plungermeans. Also disposed on opposite sides of the plunger means are a pairof rollers that transmit the reshaping force from the plunger means tothe pivotally-mounted arms. Thus, the interior passage is expanded bythe jaws in synchronization with the plunger means retracting from aposition of maximum compression of the flexible tube.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a volumetric pump in accordancewith the present invention;

FIG. 2 is an isometric view of the volumetric pump, showing an accessdoor that is closed and latched in place;

FIG. 3 is an isometric view, similar to that shown in FIG. 2, but withthe access door shown in an open position, disclosing the path followedby a flexible tube through the volumetric pump;

FIG. 4 is a longitudinal cross section of the pump assembly shown inFIGS. 2 and 3;

FIG. 5 is a schematic transverse cross section of the volumetric pump,illustrating compression of the flexible tube to pump fluid;

FIG. 6 is a schematic cross section of the volumetric pump, illustratingreshaping of the flexible tube to facilitate its filling with fluid;

FIG. 7 is a plan view illustrating the calibration of one of the tubereshaping arms to achieve a desired angular deflection;

FIG. 8 is an isometric view of an inlet cracking valve used in thevolumetric pump and a transverse section of a cam assembly that is usedto actuate the cracking valve;

FIG. 9 is an analogous view to that of FIG. 8, isometrically showing anoutlet cracking valve used in the volumetric pump and a transversesection of the cam assembly that is used to actuate the outlet crackingvalve;

FIGS. 10A-10C are cutaway, longitudinal cross sections of the volumetricpump respectively illustrating a fill segment, a pumpback-pressurizationsegment, and a pumping segment of the pumping cycle;

FIG. 11 illustrates a profile of the inlet cracking valve cam track;

FIG. 12 illustrates a profile of the outlet cracking valve cam track;

FIG. 13 illustrates a profile of the plunger cam track;

FIG. 14 is an isometric view of a portion of the cam assembly,illustrating a torque compensation track, a torque compensation followerand roller, and a cam assembly position sensor;

FIG. 15 is an exploded view isometrically illustrating one of the tubereshapers, the plunger and support flexures, and pressure sensors usedin the volumetric pump;

FIG. 16 is an isometric view of a cassette used with a differentembodiment of the volumetric pump;

FIG. 17 is a plan view of the cassette with a front panel and a flexiblemembrane removed to illustrate a fluid path through the cassette;

FIG. 18 is a cross-sectional view of the cassette, taken generally alongsection line 18--18 in FIG. 17, but showing the front panel and theflexible membrane;

FIG. 19 is a partial cross-sectional view of the inlet cracking valve inthe cassette, taken generally along section line 19--19 in FIG. 17, andshowing the front panel and the flexible membrane;

FIG. 20 is a longitudinal cross-sectional view schematicallyillustrating a volumetric pump drive mechanism for the cassette;

FIG. 21 is a plan view of the volumetric pump drive mechanism shown inFIG. 20;

FIG. 22 is a timing chart indicating the timing motor shaft revolutionsand rpm for low flow ranges of the volumetric pump; and

FIG. 23 is a schematic block diagram illustrating a volumetric pumpcontroller.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The term "volumetric pump" is applied to the present invention becauseit appropriately emphasizes one of the invention's more importantadvantages. Specifically, during each pumping stroke, the volumetricpump consistently and repeatedly displaces a defined volume of fluid ata defined pressure, thereby ensuring that a desired rate of fluid flowis accurately provided by the pump.

In FIG. 1, a volumetric pump in accordance with the present invention isgenerally illustrated in block diagram at reference numeral 30.Volumetric pump 30 comprises a number of components that are seriallyarranged along a fluid path through the pump. A liquid 31 that isadministered by volumetric pump 30 is supplied from a container 32through flexible tubing 34. Liquid 31 enters volumetric pump 30 througha proximal portion 34a of the flexible tubing. The fluid path continuesthrough a pumping portion 34b and exits the pump through a distalportion 34c of the flexible tubing. Distal portion 34c of the flexibletubing is connected to a needle/catheter 36 that is used to introduceliquid 31 output from the pump intravenously into a patient. Of course,volumetric pump 30 may also be used in other applications wherein distalportion 34c of the flexible tubing is connected to some other apparatusdisposed downstream of volumetric pump 30.

Flexible tubing 34 is continuous, but for purposes of this disclosure,is referred to as divided into the proximal, pumping, and distalportions 34a, 34b, and 34c, respectively; preferably, it comprises apolyvinyl chloride (PVC) disposable tube set, such as is customarilyused to administer fluids intravenously to a patient. The tubing mayhave a 0.137" O.D. and 0.100" I.D.

In this application of the volumetric pump, it is desirable to preventfree flow of liquid 31 from container 32 into the patient. For thisreason, volumetric pump 30 includes a free flow latch 38, which clampsproximal portion 34a of the flexible tubing to prevent liquid 31 fromcontainer 32 flowing freely into a patient, due to head pressure. Freeflow latch 38 does not restrict fluid flow during the normal pumpingoperation of volumetric pump 30, but is configured to automaticallyclamp proximal portion 34a of the flexible tubing when a door 78 (shownin FIGS. 2 and 3) on volumetric pump 30 is opened. While door 78 isclosed, free fluid flow through volumetric pump 30 is otherwiseprecluded by volumetric pump 30, as explained below. The position ofdoor 78 is sensed by a door position detector 62, producing a signalthat prevents operation of volumetric pump 30 when door 78 is open.Similarly, a tubing detector 40 is interconnected to free flow latch 38,and produces a signal indicative of the presence of flexible tubing 34within free flow latch 38; operation of volumetric pump 30 is inhibitedif the signal indicates that the flexible tubing is not in place.

A balance block 42 rests against proximal portion 34a of flexible tubing34 and serves to compensate for variations or changes in the elasticityof flexible tubing 34. The function and operation of balance block 42are more fully explained below.

Next in the serial arrangement of components along the fluid path withinvolumetric pump 30 is a proximal pressure sensor 44, which operates tosense the pressure of fluid within proximal portion 34a of the flexibletubing. Proximal pressure sensor 44 produces a signal indicative offluid pressure in this portion of flexible tubing 34 for use inmonitoring the operation of the pump and to determine if proximalportion 34a has become occluded.

A key element in the operation of volumetric pump 30 is an inletcracking valve 46, disposed immediately downstream of proximal pressuresensor 44. Inlet cracking valve 46 functions in cooperation with aplunger 48 and an outlet cracking valve 52, which are disposedsequentially downstream of the inlet cracking valve, to provide thedisplacement of a volumetric quantity of fluid from pumping portion 34bof the flexible tubing by volumetric pump 30 and to generally isolatethe volumetric pump from variations in proximal and distal fluidpressure, due, for example, to variations in the elevation of container32, or variations in the back pressure of fluid distal portion 34c ofthe flexible tubing. A flow detector 54 is interconnected with outletcracking valve 52 and produces a signal indicating whether fluid issuccessfully being pumped by volumetric pump 30 into distal portion 34c.Tubing shapers 50a and 50b are disposed at each side of plunger 48 andact to rapidly reform pumping portion 34b of the flexible tubing as itrefills with fluid during each pump cycle, insuring consistentvolumetric refill with each pumping stroke.

A distal pressure sensor 56 produces a signal indicative of the fluidpressure within distal portion 34c of the flexible tubing, i.e., theoutput pressure of volumetric pump 30. The distal fluid pressure is usedfor monitoring the operation of volumetric pump 30 and for sensing anocclusion of flexible tubing 34.

Immediately adjacent distal pressure sensor 56 is a balance block 58.Cooperating with outlet cracking valve 52, a balance block 58compensates for changes or variations in the stiffness or elasticity offlexible tubing 34, in a manner similar to that in which balance block42 cooperates with inlet cracking valve 46.

An air sensor 60 is the last component along the fluid path throughvolumetric pump 30. Air sensor 60 detects the presence of air bubbleslarger than a predefined volume in the fluid discharged from thevolumetric pump, and produces a signal indicative of such air bubbles,which stops volumetric pump 30 and initiates an alarm to prevent apotentially harmful air embolism forming in the fluid being introducedinto a patient through needle/catheter 36. Air sensor 60 comprises agenerally conventional piezoelectric ultrasonic transmitter and receiver(not separately shown), spaced apart on opposite sides of distal portion34c of the flexible tubing. The transmitter produces an ultrasonicsignal that is transmitted through flexible tubing 34 to the receiver.Liquid present in flexible tubing 34 between the transmitter andreceiver conveys the ultrasonic signal much more efficiently than doesan air bubble. The receiver produces an electronic signal in response tothe level of the ultrasonic signal reaching it, the amplitude of theelectronic signal indicating whether an air bubble or liquid is presentin flexible tubing 34 between the transmitter and receiver. Details ofair sensor 60 are not illustrated because such devices are generallywell known to those of ordinary skill in this art.

In FIGS. 2 and 3, volumetric pump 30 is illustrated in isometric view.As shown therein, volumetric pump 30 includes a molded plastic housing70, having a handle 72 on its upper surface to facilitate carrying thevolumetric pump to a point of use. A control panel 74 and a display 76are disposed on the right side of the front surface of volumetric pump30, and are respectively used by an operator for entry and display ofdata that controls the volumetric pump.

On the back of housing 70 is formed a clamp 88, which is used toremovably attach volumetric pump 30 to a post 86, for example at thebedside of a patient. Details of clamp 88 are not shown, since it isgenerally typical of those used with other types of medical apparatusintended for connection to vertical posts.

In FIG. 2, door 78 is shown latched closed, the appropriate dispositionfor use of the volumetric pump, while in FIG. 3, door 78 is shown in anopen position. A latch handle 80 is pivoted upwardly so that door 78 canbe swung open on a hinge 96, giving access to an inner cover 92 thatdefines the path followed by flexible tubing 34 through volumetric pump30. As noted above, when door 78 is opened while flexible tubing 34 isthreaded through the volumetric pump and connected to container 32, freeflow latch 38 clamps the flexible tubing closed to prevent liquid 31 incontainer 32 from free flowing through flexible tubing 34. The mechanismthat actuates free flow latch 38 when door 78 is opened is not shownsince it is not particularly relevant to the present invention.

Flexible tubing 34 is angled upwardly where it passes through an entryslot 82 formed on the side of door 78, insuring that any of liquid 31leaking from container 32 drips from a loop formed in flexible tubing 34and does not run into volumetric pump 30. After door 78 is swung open,flexible tubing 34 is readily threaded into a channel 90 defined alongthe longitudinal center of inner cover 92. An exit slot 84, formed inthe lower side portion of door 78, overlies distal portion 34c of theflexible tubing. A pressure plate 94 disposed on the inner surface ofdoor 78 comes into contact with flexible tubing 34 along the length ofchannel 90 as door 78 is closed and latched with handle 80.

FIGS. 4, 5, and 6 show details of the interior of volumetric pump 30.Pressure plate 94 defines a reference plane or surface in respect toeach of the components of volumetric pump 30 that act to compressflexible tubing 34 and is mounted so that it floats on a plurality ofhelical coiled springs 212. Springs 212 bias pressure plate 94 away fromthe inner surface of door 78. When door 78 is closed, pressure plate 94contacts inner cover 92 at several points. Helical springs 212, whichare relatively stiff, are thus slightly compressed, and thereforeaccommodate variations in the tolerances of door 78 and other relatedparts that arise during construction of volumetric pump 30. Suchtolerances might otherwise affect the position of the reference planedefined by pressure plate 94.

Most of the components comprising volumetric pump 30 are mounted on aframe 100 within housing 70. For example, frame 100 includes inletcracking valve pivot mounts 102 and outlet cracking valve pivot mounts104, about which inlet cracking valve 46 and outlet cracking valve 52respectively pivot.

Inlet cracking valve 46 contacts proximal portion 34a of the flexibletubing along a valve face 106a. Similarly, outlet cracking valve 52contacts distal portion 34c of the flexible tubing along a valve face106b. The pivotal motion of inlet cracking valve 46 and outlet crackingvalve 52 respectively varies the force with which valve faces 106a and106b contact flexible tubing 34 to control fluid flow therethrough bycompressing the flexible tubing against pressure plate 94. Plunger 48compresses pumping portion 34b of the flexible tubing against pressureplate 94 to displace fluid from within a pumping chamber defined betweenthe inlet and outlet cracking valves 46 and 52. In part becausevolumetric pump 30 includes inlet and outlet cracking valves 46 and 52,it operates differently than the prior art plunger type periostalticpumps, as will be apparent from the following disclosure.

An inlet valve arm 108 extends upwardly from valve face 106a on inletcracking valve 46. Disposed generally above inlet cracking valve pivotmounts 102 are flat metal spring flexures 110, which connect balanceblock 42 to a slot 134, formed on the back side of inlet valve arm 108.Flexures 110 are snapped within slot 134 and flex to enable inlet valvearm 108 to pivot valve face 106a away from pressure plate 94 through agreater angle that would otherwise be possible, without closing offfluid flow through flexible tubing 34 due to compression of the flexibletubing by balance block 42. Inlet cracking valve pivot mounts 102 areconnected to downwardly depending pivot arms 136 on inlet cracking valve46, at each side of flexible tubing 34, and are centered between balanceblock 42 and valve face 106a. The stiffness of flexible tubing 34 actson balance block 42 and flexures 110, and the balance force developed asa function of this stiffness (or lack of elasticity) tends to pivotinlet valve face 106a against pressure plate 94, thereby increasing theforce exerted by that part of inlet cracking valve 46 to compress theflexible tubing. The stiffness of flexible tubing 34 also resistscompression by inlet valve face 106a to a similar extent. Accordingly,variations in the elasticity of flexible tubing 34 that affect the forcerequired for inlet valve face 106a to compress the tubing areautomatically compensated for by balance block 42.

Inlet cracking valve 46 operates in three distinct modes, the forceapplied by valve face 106a to compress flexible tubing 34 beingsubstantially different in each mode. Two different spring-bias forcesact on inlet valve arm 108. A fluid flow control force is applied toinlet valve arm 108 by a flat metal spring cracking flexure 112, actingagainst a knob 114, which is disposed at one end of inlet valve arm 108.The additional force necessary to compress flexible tubing 34sufficiently to completely close off fluid flow past inlet crackingvalve 46 is supplied by a flat metal spring closure flexure 120. Closureflexure 120 acts upon a side arm 116, disposed on one side of inletvalve arm 108. The combined force provided by cracking flexure 112 andclosure flexure 120 (in addition to the balance force provided bybalance block 42) pivots inlet cracking valve 46 about a pivot axisextending through inlet cracking valve pivot mounts 102, to completelyblock fluid flow through flexible tubing 34.

An inlet valve cam follower 122 selectively determines whether crackingflexure 112 and closure flexure 120 apply force against inlet valve arm108 and thus determines the three modes in which inlet cracking valve 42operates. Inlet valve cam follower 122 includes a roller 124 rotatablymounted in a hood 126, which is attached via an inlet follower flexure128 to a plurality of blocks 130. Blocks 130 are also used in mountingcracking flexure 112 and closure flexure 120 to a bracket 135 and toprovide appropriate spacing between these flexures and bracket 135.Bolts 132 connect the ends of each of these flexures to bracket 135,which comprises a portion of frame 100.

Roller 124 rolls along an inlet valve cam track 140, disposed on arotating cam assembly 142. Cam assembly 142 turns on a camshaft 144,which at each of its ends is mounted to frame 100 in bearings 220 (seeFIGS. 5 and 6). A motor shaft 148 extends downwardly from a motor 146,and a helical gear 224 on motor shaft 148 drivingly engages gear teeth222, which are formed on one end of cam assembly 142, causing the camassembly to rotate in a clockwise direction, as viewed in FIG. 4. Theradial distance between camshaft 144 and the point where roller 124contacts the surface of inlet valve cam track 140 varies as cam assembly142 rotates, moving inlet valve cam follower 122 radially back and forthso as to control the forces applied to inlet valve arm 108.Specifically, as hood 126 is forced radially back against closureflexure 120, it lifts the closure flexure away from side arm 116,eliminating the force normally exerted by the by valve face 106a againstflexible tubing 34. In this configuration, inlet cracking valve 46 is ina "cracking mode."

As hood 126 moves further radially outward, closure flexure 120 contactsa "V-shaped" side arm 118 that is formed on the side of inlet valve arm108, causing inlet valve arm 108 to pivot valve face 106a away frompressure plate 94. In this configuration, inlet cracking valve 46 is inan open mode, wherein liquid 31 freely flows from container 32 throughproximal portion 34a of the flexible tubing and into pumping portion34b. Flexures 110 bend as valve face 106a pivots away from pressureplate 94, so that balance block 42 does not close off fluid flow throughproximal portion 34a of the flexible tubing.

When both closure flexure 120 and cracking flexure 112 are allowed toact on inlet valve arm 108, valve face 106a compresses flexible tubing34 against pressure plate 94 sufficiently to completely block fluid flowthrough the flexible tubing. In this configuration, inlet cracking valve46 is in a "closed mode."

An outlet valve cam track 150 is disposed between inlet valve cam track140 and a plunger cam track 152. Plunger cam track 152 provides asurface at varying radii about camshaft 144 for actuating plunger 48 tocompress pumping portion 34b of the flexible tubing against pressureplate 94. A roller 154 is rotatably mounted on a base 156 of olunger 48,and is thus disposed to roll along plunger cam track 152. Also mountedon base 156, at opposite sides of roller 154, are tubing shaper rollers160. The disposition of tubing shaper rollers 160 is more clearly shownin FIGS. 5 and 6, and their operation in respect to shaping flexibletubing 34 is disclosed in detail below.

As shown using hidden lines in FIG. 4, the back side of cam assembly 142includes a torque compensation track 170. A conically-shaped torquecompensation roller 172 rolls along torque compensation track 170,applying a rotational torque to cam assembly 142 that compensates for anopposite torque resulting from rapid changes in the shape of inlet valvecam track 140, outlet valve cam track 150, and plunger cam track 152.Torque compensation roller 172 is mounted on a flat metal spring torquecompensation flexure 174 that applies a biasing force to cam assembly142.

Like inlet cracking valve 46, outlet cracking valve 52 has a generally"Y-shaped" configuration and includes an outlet valve arm 180, which isconnected to outlet valve face 106b and to balance block 58. On oppositesides of flexible tubing 34, pivot arms 136 extend downwardly,connecting to outlet cracking valve pivot mounts 104 on frame 100.Balance block 58 rest on distal portion 34c of the flexible tubing anddevelops a force proportional to the stiffness (or lack of elasticity)of flexible tubing 34, which tends to increase the compression forceapplied against flexible tubing 34 by outlet valve face 106b tocompensate or balance the resistance to compression caused by thestiffness (or lack of elasticity) of the flexible tubing. Just asbalance block 42 compensates for changes or variations in elasticity ofthe flexible tubing in respect to inlet cracking valve 46, balance block58 compensates for such changes and variations in respect to outletcracking valve 52. However, since outlet cracking valve 52 is neverpivoted to an open mode like inlet cracking valve 46, balance block 58is integrally attached to outlet valve arm 180. Flexures 110 are notrequired, since the extent of pivotal rotation of outlet cracking valve52 is substantially more limited than for inlet cracking valve 46. Atall times, even when volumetric pump 30 is not pumping fluid, eitherinlet cracking valve 46 or outlet cracking valve 52 is in its closedmode, preventing liquid 31 from free flowing through flexible tubing 34.

As shown in FIG. 4, outlet cracking valve 52 is in its closed mode,compressing flexible tubing 34 against pressure plate 94 sufficiently toblock fluid flow therethrough. In this configuration, a flat metalspring cracking flexure 182 applies force against a knob 184 on the topof outlet valve arm 180. In addition, a flat metal spring closureflexure 188 applies a biasing force against a side arm 186 that extendsoutwardly from the side of outlet valve arm 180.

An outlet valve cam follower 190 includes a roller 192, which rollsalong outlet valve cam track 150. Roller 192 is rotatably mounted withina hood 194, which is connected to a flat metal spring follower flexure196. Follower flexure 196 spring biases roller 192 into contact withoutlet valve cam track 150. The lower ends of follower flexure 196,cracking flexure 182, and closure flexure 188 are all secured at blocks130 to bracket 135 by bolts 132, just as the corresponding elements arein respect to inlet cracking valve 46. As outlet valve cam follower 190follows outlet valve cam track 150, hood 194 periodically contactsclosure flexure 188, lifting it away from side arm 186 so that the flowcontrol force provided by cracking flexure 182, added to the balanceforce developed by balance block 58, is transmitted to valve face 106b,thereby compressing flexible tubing 34 against pressure plate 94 with acracking force. In this configuration, outlet cracking valve 52 is inits cracking mode.

As plunger 48 compresses pumping portion 34b of the flexible tubingagainst pressure plate 94, the pressure developed by liquid trappedbetween inlet cracking valve 46, which is closed, and outlet crackingvalve 52 acts on valve face 106b, in opposition to the cracking forceproduced by cracking flexure 182 and balance block 58. As the forcedeveloped by the fluid pressure reaches a predetermined level sufficientto cause outlet cracking valve 52 to pivot open slightly, liquid 31flows past the outlet cracking valve from pumping portion 34b of theflexible tubing. Liquid 31 is thus delivered by volumetric pump 30 at apredefined cracking pressure.

A strain gauge 198 is mounted to cracking flexure 182. Strain gauge 198develops an output signal corresponding to the stress developed incracking flexure 182, therefore indicating the pivotal motion of outletvalve arm 180 as it rotates to allow fluid flow past outlet crackingvalve 52. Accordingly, strain gauge 198 comprises flow detector 54 fordetermining whether fluid is being pumped through distal portion 34c ofthe flexible tubing as a result of displacement by plunger 48. Ifpumping portion 34b of the flexible tubing contains a relatively largeproportion of air or other compressible gaseous fluid, plunger 48 cannotdevelop sufficient fluid pressure to overcome the cracking force exertedby cracking flexure 182 and balance block 58. As a result, strain gauge198 fails to detect the pivotal motion of outlet valve arm 180,indicating that fluid flow past outlet cracking valve 52 has notoccurred during a pumping stroke of plunger 48. Accordingly, the signalfrom strain gauge 198 can be used to detect whether container 32 has rundry or whether flow of liquid 31 into volumetric pump 30 has otherwisebeen interrupted. The signal produced by strain gauge 198 is simply a"go/no-go" signal as opposed to a signal that is accurately proportionalto the movement of outlet valve arm 180. This go/no-go signal is used tostop volumetric pump 30 and initiate an alarm when the expected fluidflow is not obtained, thereby alerting medical personnel of the problemso that it can be corrected.

Instead of strain gauge 198, various other types of motion sensors maybe used to produce a signal indicative of the pivotal motion of outletvalve arm 180. For example, outlet valve arm 180 can be connected to alinear variable displacement transformer (LVDT) that uses motion toproduce a signal corresponding to a relative change in the magneticcoupling between two electromagnetic coils, or may comprise a variablecapacitor that changes capacitance value as outlet valve arm 180 pivots.Similarly, a Hall sensor or optical sensor can be used to detect pivotalmotion of outlet valve arm 180, and thus may serve as alternative typesof flow detectors.

Proximal pressure sensor 44 comprises a block 204, which is springbiased into contact with proximal portion 34a of the flexible tubing,and is disposed between inlet cracking valve 46 and balance block 42. Aspring-bias force for proximal pressure sensor 44 is provided by twopairs of longitudinally-extending flexures 202, disposed on each side ofplunger 48. Flexures 202 are connected to support plates 266 on frame100 by fasteners 206 at about the midpoint of the flexures. One of thefour flexures 202 connecting block 204 to support plates 266 includes astrain gauge 200, which responds to stress developed in that flexure 202as a function of fluid pressure within proximal portion 34a of theflexible tubing. As the fluid pressure increases within this portion offlexible tubing 34, flexures 202 connected to block 204 experienceincreased stress, producing a corresponding change in the output signalfrom strain gauge 200.

Similarly, distal pressure sensor 56 comprises a block 210, which isconnected to the other ends of flexures 202. A strain gauge 208 isdisposed on one of the four flexures, intermediate block 210 and one ofthe support plates 266. Strain gauge 208 produces a signal correspondingto the fluid pressure within distal portion 34c of the flexible tubing,based upon stress developed in flexures 202 as a result of thatpressure. Distal pressure sensor 56 can be used to determine if distalportion 34c of the flexible tubing has been kinked, interrupting fluidflow through flexible tubing 34, for example, as might occur if apatient rolled over onto flexible tubing 34. Such a condition causes anotable increase in the distal fluid pressure that triggers an alarm andshuts off volumetric pump 30.

In FIGS. 5, 6, and 7, details of tubing shapers 50a and 50b aredisclosed. Since it is preferable to use relatively low cost PVC tubingin connection with volumetric pump 30, tubing shapers 50a and 50b areprovided to ensure consistent operation and volumetric capacity ofpumping portion 34b of the flexible tubing throughout the entireoperating range of volumetric pump 30. At relatively high pumping rates,PVC tubing does not fully recover to its normal round uncompressed shapefrom a compressed condition rapidly enough to fill completely withfluid. Accordingly, the volumetric displacement of fluid within the PVCtubing that occurs with each pumping stroke is less than desired. Toavoid this problem, tubing shapers 50a and 50b force pumping portion 34bof the flexible tubing to recover rapidly to its maximum volumetriccapacity, i.e., to open sufficiently so that the desired amount ofliquid 31 fills the pumping chamber defined by pumping portion 34b ofthe flexible tubing.

Each tubing shaper 50a and 50b comprises an angled arm 234, terminatingat one end in a longitudinally-extending jaw 236. Arms 234 are attachedto frame 100 at pivot mounts 230, about which arms 234 rotate as tubingshaper rollers 160 roll along inner surfaces 232 of arms 234. Thus, thereciprocating up-and-down motion of plunger 48 along its reciprocationaxis inherently acts on tubing shaper rollers 160 in "lock-step",causing jaws 236 to pinch pumping portion 34b of the flexible tubing atthe proper time, thereby reforming flexible tubing 34 into the requiredpumping volume or capacity as plunger 48 lifts away from pressure plate94.

In FIG. 5, tubing shapers 50a and 50b are shown moving in oppositedirections, away from pumping portion 34b of the flexible tubing asplunger 48 descends to compress flexible tubing 34, displacing fluidfrom pumping portion 34b. However, in FIG. 6, plunger 48 is shown movingupwardly away from pressure plate 94, acting on tubing shaper rollers160 to force opposing jaws 236 to swing inwardly toward each other inorder to reshape pumping portion 34b of the flexible tubing so that itachieves its desired volumetric capacity.

To further enhance the repeatability and consistency of the volumetriccapacity defined in pumping portion 34b of the flexible tubing, plungercam track 152 is sized and shaped so that plunger 48 never completelycompresses pumping portion 34b of the flexible tubing, even at thelower-most point of the plunger's reciprocal stroke. In addition, at thetop of its reciprocal stroke, plunger 48 remains in contact with pumpingportion 34b of the flexible tubing. The range of diametrical compressionof flexible tubing 34 is from about 15% at the top of the pumping stroketo about 85% at the bottom of the pumping stroke of plunger 48. Sinceflexible tubing 34 need not recover to a fully uncompressed condition,i.e., to a perfect circular cross section, changes in the elasticity offlexible tubing 34 due to continued use and repeated compression havemuch less effect on the volumetric capacity of pumping portion 34b ofthe flexible tubing than would otherwise occur.

In order to calibrate tubing shapers 50a and 50b so that their range ofmotion corresponds to that required to achieve proper reshaping ofpumping portion 34b of the flexible tubing, a wedge-shaped slot 240 isprovided in the upper outer portion of arms 234. To adjust the anglebetween the upper and lower portions of each arm 234, a wedge-shapedinsert 238 is driven into wedge-shaped slot 240, deflecting the upperportion of arm 234 through an angle, as indicated by reference numeral242. Angle 242 is determined by use of an appropriate calibration jig(not shown) during manufacture of tubing shapers 50a and 50b, or duringassembly of these components in volumetric pump 30.

Details of inlet cracking valve 46 are shown in FIG. 8, and details ofoutlet cracking valve 52 are shown in FIG. 9. In these drawings, it isapparent that downwardly depending pivot arms 136 straddle flexibletubing 34, and are spaced apart sufficiently so that blocks 204 and 210of proximal pressure sensor 44 and distal pressure sensor 56 can fittherebetween. FIG. 8 more clearly illustrates side arm 116 and V-shapedside arm 118 at the top of inlet valve arm 108. In FIG. 9, the specificdisposition of side arm 186 in respect to outlet valve cam follower 190,closure flexure 188, and cracking flexure 182 is also more clearlyshown.

One of the advantages of using flat metal spring flexures, i.e.,cracking flexure 112 and closure flexure 120, for biasing inlet valvearm 108 is that the force provided by each of these flexures is muchmore readily controlled than is typically the case with other types ofspring assemblies. For example, by trimming the shape of these flexuresor selecting flexures of a different thickness, the spring force theyproduce (i.e., their spring constant, K) can be readily modified andconsistently controlled. The same advantages apply to the other flexuresused in volumetric pump 30, such as inlet follower flexure 128 andbalance block flexures 110. Accordingly, the cracking pressure and othercharacteristics of volumetric pump 30 can be precisely determined.

The consistent volumetric displacement of fluid developed by volumetricpump 30 is in part due to the free floating suspension of plunger 48provided by other flexure components. As shown in FIG. 15, plunger 48 issupported by two pairs of transversely-extending flat metal springflexures 158. The ends of flexures 158 are mounted to frame 100 viaposts 264, only a few of which are shown in FIG. 15 for clarity. The twoflexures 158 in each pair of flexures are generally parallel to eachother and are connected to plunger 48 at spaced-apart points, therebyensuring that plunger 48 is free to move along a reciprocation axis, ina direction transverse to the longitudinal axis of flexible tubing 34,but is constrained by flexures 158 to resist twisting and lateraldisplacement.

Plunger base 156 includes two long arms 260 extending longitudinally oneach side of roller 154, and two shorter arms 262, also extendinglongitudinally, but disposed below tubing shaper rollers 160. One pairof flexures 158 are connected to long arms 260, and the other pair offlexures 158 are connected to shorter arms 262. By mounting plunger 48to flexures 158 at these two different elevational positions along itsreciprocation axis, and by providing different spacing between theflexures attached to long arms 260 as compared to the spacing betweenthe flexures 158 attached to shorter arms 262, substantial rigidity isobtained in respect to possible movement by plunger 48 in alldirections, except up and down along its reciprocation axis. Eachflexure 158 readily bends elastically about an axis that extends acrossits flat surface, transverse to its longitudinal axis, but is quitestiff in resisting bending about an orthogonal axis that is normal toits flat surface. Flexures 158 are also relatively stiff and unyieldingin respect to longitudinal tension or compression forces. Theseproperties, coupled with the arrangement of flexures 158 used to supportplunger 48 provide the resistance to motion of the plunger along all butthe reciprocation axis noted above. Flexures 158 also provide a biasingforce directed along the reciprocation axis that maintains roller 154 incontact with plunger cam track 152.

FIG. 14 illustrates details of torque compensation track 170, which isdisposed on the back side of cam assembly 142. The profile ordisplacement of torque compensation track 170 varies in a directiongenerally parallel to camshaft 144, and compensates for torque developedon the other three cam tracks 140, 150, and 152 defined on cam assembly142 as the corresponding cam followers travel along rapidly changingradial pitches. A rapid radial change in the profile of one of these camtracks develops an angular torque component tending to rotate camassembly 142, which can overdrive motor 146 beyond its desired speed. Tocompensate and prevent such variation in the speed of cam assembly 142,torque compensation track 170 is profiled to develop an opposing torquethat acts on cam assembly 142. Torque compensation roller 172 has aconical shape and is mounted on torque compensation flexure 174 at anangle "A" corresponding to the cone angle of the roller to accommodatethe different rates of linear travel of torque compensation roller 172along the radially inner and outer edge of torque compensation track170. A round torque compensation roller would scrub and wear if used inplace of conical torque compensation roller 172.

As also shown in FIG. 14, a Hall sensor 176 is positioned to detect ahome position of cam assembly 142 at each of its rotations, as a magnet250 disposed in cam assembly 142 passes Hall sensor 176. FIGS. 11, 12,and 13 illustrate the profile of inlet valve cam track 140, outlet valvecam track 150, and plunger cam track 152 in respect to the homeposition, which is indicated at the bottom of each of the cam trackprofiles at 0° rotation. Each pumping cycle of volumetric pump 30corresponds to 360° of rotation of cam assembly 142 from the homeposition shown in FIGS. 11, 12, and 13.

The operation of volumetric pump 30 can be readily understood byreference to FIGS. 10A, 10B, and 10C. In these Figures, a less detailed,longitudinal schematic view of volumetric pump 30 is shown from theopposite side, as compared to FIG. 4. Thus, in FIGS. 10A-10C, fluidenters volumetric pump 30 from the left side, where proximal portion 34aof the flexible tubing is disposed, and exits toward the right, intodistal portion 34c of the flexible tubing. The advantage of viewing theoperation of volumetric pump 30 from this perspective is that therelative positions of cracking flexures 112 and 182, closure flexures120 and 188, and cam followers 122 and 190 can readily be observed inrespect to valve arms 108 and 180.

In FIG. 10A, volumetric pump 30 is shown with inlet cracking valve 46 inits open mode, wherein valve face 106a is lifted away from pressureplate 94 to permit fluid flow from container 32 into pumping portion 34bof the flexible tubing. This view corresponds to a fill segment of thepumping cycle. To achieve this configuration, cam assembly 142 rotatesto a position where roller 124 contacts inlet valve cam track 140 at itsmaximum radial distance from camshaft 144. Inlet valve cam follower 122is forced radially outward (to the left) sufficiently so that hood 126contacts closure flexure 120, forcing it away from side arm 116 and intocontact with V-shaped side arm 118, thereby pivoting inlet crackingvalve 46 counterclockwise around pivot mounts 102. In this rotationalposition, roller 154 contacts plunger cam track 152 at its minimumradial profile, permitting plunger 48 to move reciprocally to itsuppermost position, wherein the plunger maintains pumping portion 34b ofthe flexible tubing at approximately a 15% diametrical compression.Further, outlet valve cam follower 190 is disposed at a minimum radialprofile portion of outlet valve cam track 150, enabling closure flexure188 to act on side arm 186. The combined force of closure flexure 188and cracking flexure 182 pivot outlet valve arm 180 counterclockwisearound pivot mounts 104, bringing outlet valve face 106b intocompressive contact with flexible tubing 34 with enough force tocompletely close off fluid flow through the flexible tubing.

In FIG. 10B, cam assembly 142 has rotated into a pumpback-pressurizationsegment of the pumping cycle. During the pumpback-pressurizationsegment, outlet cracking valve 52 remains completely closed, as shown inFIG. 10A, while inlet cracking valve 46 is in its cracking mode. In thecracking mode, roller 124 contacts inlet valve cam track 140 at a pointthat defines an intermediate radius about camshaft 144. In thisposition, hood 126 of inlet valve cam follower 122 lifts closure flexure120 away from side arm 116 so that only cracking flexure 112 acts oninlet valve arm 108, producing most of the desired cracking force. Asdescribed above, the rest of the cracking force is developed by balanceblock 42, which provides a balance force that compensates for variationsand changes in the stiffness or elasticity of flexible tubing 34 thatmight otherwise vary the desired cracking force.

During the pumpback-pressurization segment of the pumping cycle, plunger48 descends from the top of the intake stroke, as shown in FIG. 10A, tothe top of the pumping stroke, wherein pumping portion 34b of theflexible tubing is diametrically compressed by approximately 40%. Asplunger 48 descends from the top of the intake stroke to the top of thepumping stroke, fluid pressure inside pumping portion 34b of theflexible tubing increases until it reaches a cracking pressure, at whichpoint the force developed by the fluid pressure acting upon the surfaceof valve face 106a is sufficient to overcome the cracking force, therebyopening inlet cracking valve 46 and allowing retrograde fluid flowthrough it from the pumping portion, back toward container 32. Duringthe pumpback-pressurization segment of the pumping cycle, excess fluidwithin pumping portion 34b of the flexible tubing is thus forced backinto proximal portion 34a of the flexible tubing. As the pumping segmentof the pump cycle begins, only a predefined volume of fluid is containedwithin pumping portion 34b of the flexible tubing.

Finally, during a pumping segment of the pumping cycle that isrepresented in FIG. 10C, cam assembly 142 rotates to a point whereinroller 124 contacts inlet valve cam track 140 at a minimum radius aboutcamshaft 144, such that inlet cracking valve cam follower 122 is nolonger in contact with closure flexure 120. Under this condition, bothcracking flexure 112 and closure flexure 120 act upon inlet valve arm108, producing a total force that causes valve face 106a to compressflexible tubing 34 against pressure plate 94, thereby completelyblocking fluid flow past inlet cracking valve 46 in either direction.

Meanwhile, outlet cracking valve 52 switches to its cracking mode, ashood 194 on the outlet valve cam follower 190 lifts closure flexure 188away from side arm 186 so that the closure flexure no longer applies aforce against outlet valve arm 180. In this configuration, crackingflexure 182 provides most of the predefined cracking force acting tocompress flexible tubing 34 against pressure plate 94 at outlet valveface 106b. Balance block 58 provides the remainder of the predefinedcracking force, compensating for variations in the stiffness orelasticity of flexible tubing 34, and thereby preventing such variationsfrom affecting the desired predefined cracking force. Plunger 48continues to descend, further compressing pumping portion 34b of theflexible tubing. Fluid pressure within the pumping portion is already atthe desired cracking pressure from the pumpback-pressurization segmentof the pumping cycle, and this cracking pressure acts on the surface ofvalve face 106b, immediately creating a force that exceeds the crackingforce of outlet cracking valve 52. The cracking pressure of the fluid(liquid 31) causes outlet cracking valve 52 to pivot clockwise aboutpivot mounts 104 sufficiently to enable fluid flow into distal portion34c of the flexible tubing. Plunger 48 continues to descend until itreaches approximately 85% diametrical compression of pumping portion 34bof the flexible tubing. At this point, a predefined volume of fluid,e.g., 100 microliters, at a predefined cracking pressure has beendisplaced from volumetric pump 30 into distal portion 34c of theflexible tubing.

From the preceding explanation, it should be apparent that each pumpingcycle of volumetric pump 30 includes three distinct segments: (1) a fillsegment during which a pumping chamber defined between inlet crackingvalve 46 and outlet cracking valve 52, i.e., the volume within pumpingportion 34b of the flexible tubing, fills with fluid; (2) apumpback-pressurization segment, wherein excess fluid within the pumpingportion of the flexible tubing is forced back into proximal portion 34aof the flexible tubing, toward container 32 as the fluid is pressurizedto the cracking pressure; and (3) a pumping segment, wherein fluidwithin the pumping portion of the flexible tubing at the crackingpressure is forced from volumetric pump 30 into distal portion 34c ofthe flexible tubing. Each of these pumping cycle segments is separatedfrom the next by a short dwell period. In the preferred embodiment ofvolumetric pump 30, each pumping cycle, i.e., each revolution of camassembly 142, corresponds to 24 revolutions of motor shaft 148. Thus,each revolution of motor shaft 148 corresponds to a 15° rotation of camassembly 142. Table I lists the revolutions of motor shaft 148 anddegrees of rotation of cam assembly 142 for each portion of the pumpingcycle.

                  TABLE I                                                         ______________________________________                                        Pumping Cycle Segment                                                                       Deg. of Rotation                                                                           Motor Shaft Revs.*                                 ______________________________________                                        Pumping       135          9                                                  Dwell         25           2                                                  Fill (intake) 110          7                                                  Dwell         20           1                                                  Pumpback-Pressuriz.                                                                         45           3                                                  Dwell         25           2                                                  ______________________________________                                         *In Table I, the revolutions of motor shaft 148 corresponding to degrees      rotation of cam assembly 142 are approximate, having been rounded to          integer numbers.                                                         

Motor 146 includes a Hall sensor (not shown) that is used to monitor therotational position of motor shaft 148. Also, in the preferred form ofvolumetric pump 30, motor 146 incorporates an optical encoder (notshown) providing 100 increments of rotational resolution, one incrementof which corresponds to a home position of motor shaft 148. However,since each rotation of cam assembly 142 corresponds to 24 revolutions ofmotor shaft 148, Hall sensor 176, which responds to magnet 250 on theback of cam assembly 142, is used to roughly define the home position ofthe camshaft, subject to the finer resolution for home positiondetermined by either the Hall sensor or optical encoder incorporated inmotor 146. Hall sensor 176 need only define home position within ±7.5°,since true home position (referred to as "home/home" position) isdetermined internally within motor 146.

Volumetric pump 30, in its preferred form, is capable of pumping fluidat rates from 1-999 ml/hr. Fluid flow rates in the range 175-999 ml/hrare achieved by varying the rotational rate of motor 146. In thepreferred embodiment, motor 146 comprises a DC brushless motor capableof speeds over 4,000 rpm; however, other types of motors may also beused. To achieve flow rates between 1 and 174 ml/hr, motor 146 isoperated in one of four separate ranges, identified in FIG. 22 byreference numerals 270, 272, 274, and 276. In each of these ranges,motor 146 is stopped during the pumping segment of the pumping cycle fora different number of intervals, and for varying amounts of time at eachinterval. For example, to achieve 174 ml/hr as shown in range 270, motor146 operates at 722 rpm during the pumping segment, and stops for 0.46second intervals between each of three 2-revolution portions of thepumping segment. To achieve 75 ml/hr in the same range, motor 146 againruns at the same speed, but stops for 0.76 second intervals between eachof the 2-revolution portions of the pumping segment. During each of thelow flow rate ranges 270-276, the remainder of the pumping cycle iscompleted at 1,011 rpm, including two revolutions past the homeposition, which carry the pumping cycle into the next pumping segment.On FIG. 22, the dashed line identifying home/home (in the upper rightcorner) refers to the true home position for both motor 146 and for camassembly 142, as described above.

Turning now to FIG. 23, a controller for volumetric pump 30 is showngenerally at reference numeral 280. Controller 280 includes amicroprocessor CPU and memory block 282 in which software algorithmsthat control volumetric pump 30 are implemented in response to inputcontrol data provided by an operator on a keypad 286. For example, anoperator might enter specific times during which a prescribed volume ofliquid 31 should be administered intravenously to a patient at aprescribed flow rate by volumetric pump 30. The control data entered viakeypad 286 are shown on display 76.

In accord with the control data entry provided on keypad 286, CPU andmemory block 282 actuates the motor at the required time, controls it toadminister the prescribed volume of liquid 31 at the prescribed rate,and keeps track of its progress through the pumping cycle in respect tofeedback signals provided by the Hall effect sensor and optical encoderbuilt into motor 146, as indicated in a block 290. In addition, each ofa plurality of sensors, including proximal pressure sensor 44, distalpressure sensor 56, flow detector 54, tubing detector 40, door positiondetector 62, and air sensor 60, listed in a sensor block 288 providesignals to CPU and memory block 282 that are used to determine thestatus of volumetric pump 30 and potentially harmful conditions. Powerfor controller 280 is supplied by a conventional power supply 294. Inthe event that any of the signals provided by the sensors in block 288indicate a potentially harmful condition, such as a large air bubble orlack of fluid flow from volumetric pump 30, CPU and memory block 282effects an alarm condition, causing both a visual and audible signal tobe generated by an alarm 292, to alert medical personnel of the problem.

For purposes of recording patient history and to interface with othercontrollers, a data output path 296 is also optionally provided for CPUand memory block 282.

Many of the desirable characteristics of volumetric pump 30 can beachieved in a different embodiment of the volumetric pump showngenerally at reference numeral 400 in FIGS. 20 and 21. A cassette foruse in connection with volumetric pump 400 is generally illustrated atreference numeral 300 in FIGS. 16-19. Cassette 300 comprises a housing302 sealed to a front panel 304, preferably made of rigid plastic. Atthe bottom of housing 302 are an inlet port 306 and an outlet port 308.Flexible tubing 310 is connected to inlet port 306 and may connect tocontainer 32. Flexible tubing 312 is connected to outlet port 308 andmay connect to needle/catheter 36 or other apparatus. A proximalair-in-line sensor 314 and a distal air-in-line sensor 316 are providedthat engage flexible tubing 310 and 312, respectively, to detect thepresence of air bubbles larger than a predetermined size within thetubing. In front panel 304 of cassette 300 are a plurality of ports,including an inlet valve port 320, a proximal pressure sensor port 324,a plunger port 330, an outlet valve port 334, and a distal pressuresensor port 338. These ports give access to a flexible membrane 340,which is disposed between front panel 304 and housing 302, overlying aplurality of fluid passages and chambers, as described below.

Cassette 300 is considered disposable and is used with volumetric pump400 as described below. An inlet valve actuator 318 on volumetric pump400 applies a variable biasing force to deflect flexible membrane 340within inlet valve port 320. In proximal pressure sensor port 324, thepressure developed by fluid behind flexible membrane 340 acts upon aproximal pressure sensor rod 322, permitting the proximal fluid pressureto be sensed.

Plunger port 330 is relatively larger than the other ports, toaccommodate a plunger 326, which is attached to a plunger rod 328extending from volumetric pump 400. Plunger 326 deflects flexiblemembrane 340 to displace fluid behind the flexible membrane as describedbelow. An outlet valve actuator 332 on volumetric pump 400 acts onflexible membrane 340 to control fluid flow behind the flexible membraneat outlet valve port 334, and a distal pressure sensor rod 336 detectsthe distal fluid pressure behind flexible membrane 340 at distalpressure sensor port 338.

In FIG. 17, a cross-sectional plan view showing the fluid passagesbehind flexible membrane 340 illustrates the fluid flow path throughcassette 300. Fluid entering inlet port 306 flows through an inletpassage 342 and through an entry port 344 at the bottom center of aproximal pressure chamber 346. Proximal pressure chamber 346 is, ofcourse, disposed behind flexible membrane 340 in the area defined byproximal pressure sensor port 324. A sealing ridge 348 defines theperimeter of proximal pressure chamber 346 and each of the otherchambers and passages within cassette 300. Flexible membrane 340 istrapped between the top of sealing ridge 348 and the back surface offront panel 304, forming a seal that prevents fluid leakage from thesechambers and passages.

A connecting passage 350 leads from proximal pressure chamber 346 intoan outer channel 352, which is disposed behind inlet valve port 320. Aninner ridge 354 separates outer channel 352 from a cracking chamber 356that is disposed in the center of inlet valve port 320. As shown inFIGS. 18 and 19, inner ridge 354 is slightly lower in elevation thansealing ridge 348 and includes a nib 378 centered on its top surface.Sealing ridge 348 connects to inner ridge 354 and extends in an inclinedown to the lower elevational level of inner ridge 354 at the opening toa connecting passage 358. Connecting passage 358 leads into a pumpingchamber 360, disposed behind plunger port 330.

From pumping chamber 360, a connecting passage 362 leads into a crackingchamber 364 at the center of outlet valve port 334. Surrouding crackingchamber 364 is an inner ridge 366, also including a nib 378 centered onits upper surface. Inner ridge 366 is slightly lower in elevation thansealing ridge 348. An outer channel 368 surrounds inner ridge 368 and isin fluid communication with a distal pressure chamber 372, which isdisposed behind distal pressure sensor port 338. From the center ofdistal pressure chamber 372, an exit port 374 leads back into a passage376, which runs along housing 302 and is in fluid communication withoutlet port 308.

In FIG. 19, inlet valve actuator 318 is shown deflecting flexiblemembrane 340 sufficiently so that fluid flow over the top of nib 378 isblocked, shutting off fluid communication between central crackingchamber 356 and outer channel 352. Clearly, if the force exerted byinlet valve actuator 318 is greater than the force developed by fluidpressure within cracking chamber 356, fluid cannot flow from crackingchamber 356 into outer channel 352 past nib 378. Under thiscircumstance, inlet valve actuator 318 completely blocks fluid flow.However, if inlet valve actuator 318 provides a predefined crackingforce to seal flexible membrane 340 against nib 378, fluid pressurewithin cracking chamber 356 may increase to a cracking pressure,exerting sufficient force against inlet valve actuator 318 that fluid isforced over nib 378 into outer channel 352. The same considerationapplies in respect to outlet valve actuator 332 and its action onflexible membrane 340 at outlet valve port 334. Also, if inlet valveactuator 318 is drawn away from flexible membrane 340, fluid may freelyflow from outer channel 352 over the top of nib 378 into crackingchamber 356.

During a filling segment of the pumping cycle for cassette 300, fluidflows freely into pumping chamber 360. During a pumpback-pressurizationsegment of the pumping cycle, inlet valve actuator 318 depressesflexible membrane 340 with the predefined cracking force, while plunger326 depresses flexible membrane 340 in the pumping chamber, forcingexcess fluid in the pumping chamber to flow past nib 378 and backthrough inlet port 306. In this pumpback-pressurization segment of thepumping cycle, outlet valve actuator 332 fully seals flexible membrane340 against nib 378 at outlet valve port 334.

In the pumping segment of the pumping cycle, inlet valve actuator 318applies a sealing force to fully close off fluid flow over nib 378 atinlet valve port 320, while outlet valve actuator 332 applies thepredefined cracking force to flexible membrane 340 at the outlet valveport 334. Fluid is thus displaced from pumping chamber 360, flows overinner ridge 366, and through outlet port 308.

Volumetric pump 400 is generally unaffected by variations in fluidpressure at inlet port 306 or outlet port 308 of cassette 300 anddelivers an accurate volumetric quantity of fluid at the desiredcracking pressure, with each pumping stroke. Furthermore, since air orother gaseous fluid trapped within pumping chamber 360 cannot becompressed sufficiently to develop the cracking pressure, volumetricpump 400 is incapable of forcing fluid to flow through the outlet portif a substantial portion of the fluid within pumping chamber 360 is airor other compressible gas.

In FIGS. 20 and 21, volumetric pump 400 for cassette 300 is shown, andis in many ways similar to volumetric pump 30. Volumetric pump 400comprises a frame 402, only a portion of which is shown. Frame 402 isadapted to mount cassette 300 in a fixed position and to applyappropriate cracking and closure forces through inlet valve actuator 318and outlet valve actuator 332 in cooperation with the reciprocatingmotion of plunger rod 328, in accordance with a defined pumping cycle,to effect fluid flow through cassette 300. To simplify the disclosure ofvolumetric pump 400, distal and proximal pressure sensors that areconnected to proximal pressure sensor rod 322 and distal pressure sensorrod 336 are not shown; however, it will be understood by those ofordinary skill in the art that proximal and distal pressure sensor rods322 and 336 can readily be connected to pressure sensors such as straingauges to effect sensing of fluid pressure within volumetric cassette300.

A motor 404 is attached to frame 402 of volumetric pump 400 and includesa motor shaft 406 connected to apply a driving force to gear teeth 412formed on one end of a cam assembly 408, via a helical gear 410. Camassembly 408 rotates around a camshaft 414 that is mounted in bearings472.

An inlet valve cam follower 416 in volumetric pump 400 includes a roller420 mounted in a hood 422. Roller 420 rolls along an inlet cam track 417formed on cam assembly 408. Inlet valve cam follower 416 is connected toa follower flexure 418, which provides a biasing force tending to keeproller 420 in contact with inlet cam track 417.

An inlet valve arm 424 is connected to inlet valve actuator 318 at apivot pin 428, and is rotatably connected to a frame member 425 at apivot mount 426. Acting on inlet valve arm 424 are a closure flexure 430and a cracking flexure 432, which together combine to provide a forcesufficient to fully close off fluid flow to and from inlet crackingchamber 356 in cassette 300. Closure flexure 430 acts upon a side arm438 disposed on the side of inlet valve arm 424. Also disposed on theside of inlet valve arm 424 is a "V-shaped" arm 440. Closure flexure430, cracking flexure 432, and inlet follower flexure 418 are allmounted to frame 402 on blocks 434, using bolts 436.

Cam assembly 408 also includes an outlet cam track 446 and a plunger camtrack 442. Rotatably mounted at the end of plunger rod 328 is a roller444, which rolls along plunger cam track 442 to reciprocally moveplunger rod 328 and actuate plunger 326, thereby displacing fluid frompumping chamber 360. Plunger rod 328 is mounted on a base 480 havinglong longitudinally-extending arms 482 and shorterlongitudinally-extending arms 486. Connected to long arms 482 andshorter arms 486 are two pairs of transversely-extending flexures 484.Flexures 484 are connected to frame 402 at each end and support plungerrod 328 just as flexures 158 support plunger 48 in volumetric pump 30.Flexures 484 also provide a biasing force to keep roller 444 in contactwith plunger cam track 442.

An outlet valve cam follower 448 includes a hood 454, in which a roller450 is rotatably mounted that rolls along outlet cam track 446. Roller450 is biased against outlet cam track 446 by an outlet follower flexure452. An outlet valve arm 464 is mounted to rotate about a pivot mount466 and is pivotally connected to outlet valve actuator 332 at a pivotpin 468. A closure flexure 456 rests against a side arm 458 formed onthe side of outlet valve arm 464, and in combination with a biasingforce provided by a cracking flexure 460, provides sufficient force tostop fluid flow between outlet cracking chamber 364 and outer channel368. Outlet valve cam follower 448 is actuated by outlet cam track 446so that hood 454 deflects closure flexure 456 away from side arm 458. Inthis configuration, only cracking flexure 460 applies a cracking forceon flexible membrane 340 through outlet valve actuator 332. Accordingly,fluid flows past inner ridge 366 when the pressure inside outletcracking chamber 364 produces a force acting on flexible membrane 340that exceeds the predefined cracking force of the outlet valve actuator.Outlet follower flexure 452, closure flexure 456, and cracking flexure460 are each mounted at blocks 435 to frame 402 using bolts 436.Attached to cracking flexure 460 is a strain gauge 470 for detectingmotion of outlet valve arm 464, thereby detecting fluid flow fromcassette 300. Other types of sensors may be used to detect motion ofoutlet valve arm 464, as described in respect to outlet valve arm 180 onvolumetric pump 30.

Rotation of cam assembly 408 effects sequential operation of volumetricpump 400, placing inlet valve arm 424 in an open mode, wherein plunger326 retracts away from flexible membrane 340, allowing fluid to fillpumping chamber 360. Thereafter, the pumpback-pressurization and pumpingsegments of the pumping cycle proceed generally as described in respectto volumetric pump 30. It should be apparent that controller 280 (FIG.23) is equally applicable to operation of volumetric pump 400 to pumpfluid through cassette 300; however, fewer sensors are required insensor block 288.

While the present invention has been disclosed in respect to preferredembodiments and modifications thereto, those of ordinary skill in theart will understand that further modifications may be made within thescope of the claims that follow. Accordingly, it is not intended thatthe claims in any way be limited by the disclosure of these preferredembodiments, but that the scope of the invention be determined entirelyby reference to the claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. Apparatus for supportinga reciprocating plunger in a positive displacement pump, comprising:a. aframe having two spaced-apart members disposed on opposite sides of theplunger; b. a plurality of pairs of flexures extending between the twospaced-apart members and the plunger, said flexures having longitudinalaxes, each flexure in a pair of flexures being aligned substantiallyparallel to the other flexure in that pair, whereby the plunger issupported by the pairs of flexures so that it can reciprocate along areciprocation axis that is generally normal to the longitudinal axes ofeach pair of flexures, but is constrained by the pairs of flexures sothat it does not move transversely in respect to the reciprocation axis.2. The apparatus of claim 1, wherein the flexures are each attached tothe spaced-apart members and to the plunger and apply a biasing force tothe plunger directed along the reciprocation axis.
 3. The apparatus ofclaim 2, wherein a first pair of the flexures are attached to theplunger at a first level along the reciprocation axis, and a second pairof flexures are attached to the plunger at a second level along thereciprocation axis.
 4. The apparatus of claim 2, wherein a first pair ofthe flexures are spaced apart a first distance, and a second pair of theflexures are spaced apart a second distance that is substantiallydifferent than the first distance.
 5. The apparatus of claim 1, whereinthe flexures bias the plunger along the reciprocation axis against adrive force that displaces the plunger along the reciprocation axis. 6.The apparatus of claim 1, wherein one pair of flexures is spaced apartfrom another pair so as to prevent rotation of the plunger about arotation axis that is normal to the reciprocation axis.
 7. The apparatusof claim 1, wherein each flexure comprises an elongate, flat metalspring.
 8. A method of constraining a plunger to move only along areciprocation axis, preventing it from twisting or moving laterally awayfrom the reciprocation axis, comprising the steps of:a. supporting theplunger with a first pair of flexible members that are substantiallyparallel and extend generally transversely to the reciprocation axis;and b. supporting the plunger with a second pair of flexible membersthat are substantially parallel and extend generally transversely to thereciprocation axis, said second pair of flexible members being spacedapart from said first pair of flexible members so that the plunger issupported thereby at spaced-apart points along the reciprocation axis,thereby preventing the plunger from twisting and moving laterally awayfrom the reciprocation axis, yet allowing it to move along thereciprocation axis.
 9. The method of claim 9, further comprising thestep of spacing the first pair of flexible members apart by asubstantially different distance than the second pair of flexiblemembers to more effectively resist twisting of the plunger.
 10. Themethod of claim 9, further comprising the steps of connecting theflexible members to opposed frame members that are spaced apart onopposite sides of the plunger, and applying a biasing force to theplunger with the flexible members, directed along the reciprocationaxis.